Manufacturing process on chromate-coated lead-containing galvanized steel sheet with anti-black patina property and anti-white rust property

ABSTRACT

In order to manufacture a chromate-coated lead-containing galvanized steel sheet having beautiful spangles and excellent anti-black patina and anti-white rust properties without relying upon any flushing treatment with Ni, etc., a lead-containing galvanized steel sheet obtained by plating in a galvanizing bath containing appropriate amounts of Pb and Al, and having beautiful spangles is coated with a chromate coating solution containing trivalent and hexavalent chromium ions in a molar ratio of 1/9 to 1/1, and nitrate ions in a molar ratio of 0.1 to 1.6 to the total of the chromium ions, preferably after treatment with an aqueous alkali solution having a pH of 9 or above, and washing with water, and the sheet is dried at an appropriate sheet temperature without being washed with water, whereby a chromate film having a coating weight of 5 to 50 mg/m 2  is formed thereon. The lead and aluminum with which the galvanized surface is enriched are, as a result, removed effectively, thereby enabling excellent anti-black patina and anti-white rust properties to be obtained.

TECHNICAL FIELD

[0001] This invention relates to a process for manufacturing achromate-coated lead-containing galvanized steel sheet having excellentanti-black patina and anti-white rust properties, and more particularly,to a process which is useful for the chromate coating of alead-containing galvanized steel sheet having beautiful spangles.

BACKGROUND ART

[0002] As the galvanizing of steel sheet is the most effective andeconomical means for protecting it from corrosion by a galvanic action,ten millions of tons of crude steel, which correspond to about 10% ofthe yearly crude steel production of Japan (about 100 millions of tons),are used for the manufacture of galvanized steel sheets which are usedin a wide variety of fields including their use as building materials,and as materials for automobiles and electric appliances. Zinc manifestsa galvanic action as described below. The two metals, zinc and iron,contacting each other form a cell in which zinc, which is the basermetal, forms the anode, while iron becomes the cathode. Thus, itrestrains any anodic dissolution by a local cell as formed by ironalone, and thereby prevents its corrosion. This rust-preventing actionends upon loss of all of zinc contacting iron, and in order to sustainthe action for a long period of time, it is necessary to restrain thecorrosion of the zinc layer, and it is, therefore, common practice tocoat the zinc layer with chromate.

[0003] The chromate coating of galvanized steel sheet has, however, thedrawback of having a black patina formed on the sheet during its storageor transport and impairing its appearance seriously, though it maydrastically improve its corrosion resistance (anti-white rust property).It is known that a black patina is likely to appear on, among others,galvanized steel sheet subjected to skin pass after galvanizing, orproduced in a galvanizing bath containing lead, or coated with a zinclayer containing several percent of aluminum.

[0004] The black patina is characterized by a grayish black colorpresented by spangles formed in a galvanized surface and having aspecific crystalline orientation, and it is, therefore, effective tominimize the spangles by a known method to restrain the appearance of ablack patina to some extent. It is also known that the use of agalvanizing bath containing only a very small amount of lead (Pb notexceeding 0.01% by weight) makes a black patina less likely to appear,since the spangles in which a black patina appears contain leadparticles forming the active sites which promote the appearance of ablack patina. The addition of lead to a galvanizing bath is, however,unavoidable, since many users of galvanized steel sheets like spangles.

[0005] According to pages 939 to 946 of TETSU & HAGANE, Vol. 77 (1991),published by The Japan Iron and Steel Association, the spangles areclassified into seven types, i.e. fern I, fern II, mirror, frost, halffern, feather, and triangle types, and the frost type spangles areparticularly likely to be enriched with Pb and Al. The users who likespangles generally prefer a surface having many frost type spangles aspresenting a beautiful appearance, but a black patina is particularlylikely to appear in frost type spangles, since they are enriched with Pband Al, as stated above.

[0006] Japanese Patent Application Laid-Open No. Sho 59-177381 proposedflashing treatment with an aqueous solution containing Ni or Co ions(for the chemical deposition of a very small amount of metal) as amethod of preventing a black patina from appearing after chromatetreatment, and the flashing treatment has recently come to be consideredas an effective method of preventing a black patina from appearing afterchromate treatment. According to the disclosure of Japanese PatentApplication Laid-Open No. Sho 59-177381, the surface of a zinc orzinc-alloy plated steel sheet is given flashing treatment with anaqueous solution having a pH of 1 to 4, or 11 to 13, and containing Nior Co ions, or both prior to its chromate treatment, whereby the metalions are deposited in metallic or oxide form on the sheet surface, andafter it is washed with water, a chromate film is formed thereon.

[0007] There is not yet any definite opinion about the mechanism whichenables the flashing treatment of a zinc or zinc-alloy plated steelsheet with Ni or Co to prevent a black patina from appearing thereonafter its chromate treatment, but according to the statement on pages150 and 151 of the Preprint for the 60th Scientific Lecture Meeting ofthe Association of Metal Surface Technology, the metal as deposited byflashing is mostly found in the grain boundary of zinc crystals, and thechromium compound as deposited by the subsequent chromate coatingtreatment is likewise distributed in the grain boundary, and it can,therefore, be presumed that some interaction occurs between the metal asdeposited by flashing and the chromium compound, and causes the latterto be adsorbed and fixed to the former.

[0008] The black patina formed on a zinc or zinc-alloy plated steelsheet looks black, apparently because the basic zinc carbonate of whichit is composed, and which is represented as (ZnCO3)x·[Zn(OH)2]y, likewhite rust, has a particle diameter falling within the visible lightwavelength range of 400 to 700 nm, and is, therefore, very likely toscatter and absorb light. The black patina is considered as a product ofcorrosion formed in an environment lacking oxygen, and particularly withthe progress of corrosion from the grain boundary. Accordingly, it isconsidered that the chromium compound with which the grain boundary isenriched by the metal deposited by flashing restrains the corrosion fromthe grain boundary and thereby contributes to preventing the formationof a black patina. Thus, the flashing treatment of a zinc or zinc-alloyplated steel sheet with Ni, Co, etc. prior to its chromate treatment canbe an effective means for preventing the formation of any black patinathereon.

[0009] It has, however, been found that the flashing treatment carriedout prior to chromate treatment makes white rust more likely to form,though it may restrain the formation of a black patina. This isapparently due to the fact that Ni or Co as deposited on the galvanizedsurface by the flashing treatment forms a local cell with zinc.

[0010] It is, therefore, an object of this invention to provide aprocess which can manufacture a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties without relying upon any flashing treatment with Ni, Co,etc., and more particularly, a chromate-coated lead-containinggalvanized steel sheet which hardly has any black patina formed thereon,even if it may have many spangles of the frost type which is liked bymany users.

DISCLOSURE OF THE INVENTION

[0011] As a result of a wide range of experiments and study which wehave made to explore the possibility of improving the anti-black patinaand anti-white rust properties of a chromate-coated lead-containing zincor zinc-alloy plated steel sheet having beautiful spangles, we, theinventors of this invention, have found that it is possible tomanufacture a chromate-coated lead-containing galvanized steel sheethaving beautiful spangles and yet showing excellent anti-black patinaand anti-white rust properties on an industrially steady basis if alead-containing zinc or zinc-alloy plated steel sheet produced in agalvanizing bath having a specific composition is treated with achromate coating solution having a specific composition and containingnitrate ions.

[0012] We have, moreover, found that it is possible to manufacture achromate-coated lead-containing galvanized steel sheet having anoutstandingly high corrosion resistance by pre-treating the sheet withan aqueous alkali solution before its treatment with the chromatecoating solution having a specific composition as mentioned above, or bygiving it two steps of treatment consisting of its pre-treatment withthe aqueous alkali solution and its treatment with the chromate coatingsolution having a specific composition and containing nitrate ions.

[0013] This invention is based on what we have found as stated above,and its salient features are as follows:

[0014] [1] A process for manufacturing a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties, characterized in that a lead-containing galvanizedsteel sheet obtained by plating a steel sheet in a galvanizing bathcontaining 0.05 to 0.3% by weight of Pb and 0.1 to 0.3% by weight of Alis coated with a chromate coating solution containing hexavalent andtrivalent chromium ions and nitrate ions in such proportions that thetrivalent chromium ions have a molar ratio of 1/9 to 1/1 to thehexavalent chromium ions, while the nitrate ions have a molar ratio of0.1 to 1.6 to the total of the chromium ions, and that the sheet isdried at a sheet temperature of 40-250° C. without being washed withwater, whereby a chromate film having a coating weight of 5 to 50 mg/m²in terms of metallic chromium is formed thereon.

[0015] [2] A process for manufacturing a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties as set forth at [1] above, wherein the lead-containinggalvanized steel sheet is obtained by employing a temperature of440-500° C. for the galvanizing bath and a temperature of 440-520° C.for the sheet to be immersed into the bath.

[0016] [3] A process for manufacturing a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties as set forth at [1] or [2] above, wherein the chromatecoating solution contains one or more kinds of metal ions selected fromamong cobalt, nickel, strontium and barium ions, and having a totalmolar ratio of 0.04 to 0.2 to the total of the chromium ions.

[0017] [4] A process for manufacturing a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties as set forth at [1] or [2] above, wherein the chromatecoating solution contains cobalt ions having a molar ratio of 0.04 to0.2 to the total of the chromium ions.

[0018] [5] A process for manufacturing a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties, characterized in that a lead-containing galvanizedsteel sheet obtained by plating a steel sheet in a galvanizing bathcontaining 0.05 to 0.3% by weight of Pb and 0.1 to 0.3% by weight of Alis treated with an aqueous alkali solution having a pH of 9 or above,and that after it is washed with water, it is coated with a chromatecoating solution containing hexavalent and trivalent chromium ions andnitrate ions in such proportions that the trivalent chromium ions have amolar ratio of 1/9 to 1/1 to the hexavalent chromium ions, while thenitrate ions have a molar ratio of 0.1 to 1.6 to the total of thechromium ions, and that the sheet is dried at a sheet temperature of40-250° C. without being washed with water, whereby a chromate filmhaving a coating weight of 5 to 50 mg/m² in terms of metallic chromiumis formed thereon.

[0019] [6] A process for manufacturing a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties as set forth at [5] above, wherein the lead-containinggalvanized steel sheet is obtained by employing a temperature of440-500° C. for the galvanizing bath and a temperature of 440-520° C.for the sheet to be immersed into the bath.

[0020] [7] A process for manufacturing a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties as set forth at [5] or [6] above, wherein the chromatecoating solution contains one or more kinds of metal ions selected fromamong cobalt, nickel, strontium and barium ions, and having a totalmolar ratio of 0.04 to 0.2 to the total of the chromium ions.

[0021] [8] A process for manufacturing a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties as set forth at [5] or [6] above, wherein the chromatecoating solution contains cobalt ions having a molar ratio of 0.04 to0.2 to the total of the chromium ions.

[0022] According to this invention, a chromate-coated lead-containinggalvanized steel sheet which is excellent in all of its film appearance,and anti-black patina and anti-white rust properties can be manufacturedsteadily from a lead-containing galvanized steel sheet having beautifulspangles without relying upon any flashing treatment thereof with Ni,Co, etc.

BEST MODE OF CARRYING OUT THE INVENTION

[0023] This invention is a process for the chromium coating of alead-containing galvanized steel sheet obtained by plating in agalvanizing bath containing 0.05 to 0.3% by weight of Pb and 0.1 to 0.3%by weight of Al. The lead and aluminum in the galvanizing film on thelead-containing galvanized steel sheet are the constituents necessaryfor forming a beautiful pattern of spangles, and improving the adhesionof the film, respectively. The galvanizing film has a surface enrichedwith such lead and aluminum, but its surface enriched with lead makesthe film lack electrochemical uniformity, and thereby promotes theformation of a black patina. The enrichment of the film surface withlead is likely to occur particularly to the surface having frost typespangles as obtained by galvanizing under specific conditions (i.e. aspecific temperature of the galvanizing bath and a specific temperatureof the steel sheet upon immersion into the bath). The aluminum stays inmost of the cases as a stable oxide in the skin of the galvanizing film,but as the aluminum oxide does not easily react with a chromate coatingsolution, it makes it difficult to form a uniform chromate film andthereby achieve an improved corrosion resistance.

[0024] In connection with these problems, we have found that, if alead-containing galvanized steel sheet is brought into contact with achromate coating solution containing nitric acid, it is possible toremove lead from its galvanized surface and thereby restrain theformation of a black patina effectively. Although even such chromatetreatment may, of course, not be able to remove all of the lead from thegalvanized surface, but may allow some lead to remain therein, we havefound that such chromate treatment is remarkably effective inrestraining the formation of a black patina, and that a chromate filmthereby formed provides an improved corrosion resistance, too, since alarge amount of chromium covers the surface of active lead remaining inthe galvanized surface. The effective removal of the lead enriching thegalvanized surface is achieved particularly by the chromate coatingsolution containing nitric acid, and no satisfactory removal of suchlead can be obtained by any chromate coating solution containing anyother inorganic acid, such as phosphoric, sulfuric or fluoric acid.

[0025] No satisfactory removal of aluminum from the galvanized surfacecan be achieved by its treatment with any chromate coating solutioncontaining chromic acid and an inorganic acid (e.g. nitric, phosphoric,sulfuric or fluoric acid) as mentioned above. As a result of furtherstudy, we have found that the treatment of the galvanized surface withan aqueous alkali solution prior to its chromate treatment makes itpossible to remove any aluminum oxide from it and thereby improve itscorrosion resistance effectively. We have, thus, ascertained that thelead and aluminum staying in the galvanized surface in an enrichingform, which can hardly be removed by any known chromic acid, orpartially reduced chromic acid solution, can be removed adequately bytwo steps of treatment consisting of its pre-treatment with an aqueousalkali solution and its chromate treatment with a chromate coatingsolution containing nitric acid as mentioned above to thereby improveits anti-black patina property and corrosion resistance effectively.

[0026] We have further found that, if the chromate treatment with achromate coating solution containing nitric acid as stated above iscarried out by employing a chromate coating solution containinghexavalent and trivalent chromium ions and nitrate ions, and having amolar nitrate ion ratio of 0.1 to 1.6 to the total of the chromium ions,it is possible to remove lead effectively by a very small amount ofetching, and that such a chromate coating solution can be used for along time without having any sludge formed by the inclusion of zinc, orother impurity ions, but staying stable in composition.

[0027] The following is a detailed description of the invention and thereasons for the various limitations employed for defining it.

[0028] The lead-containing galvanized steel sheet for which the chromatetreatment of this invention is intended is obtained by plating a steelsheet in a galvanizing bath containing 0.05 to 0.3% by weight of Pb and0.1 to 0.3% by weight of Al, and is particularly likely to present theproblem of a black patina, as stated before. If the lead content of thebath is lower than 0.05% by weight, no satisfactorily beautiful form ofregular spangles can be produced on the galvanized steel sheet, but ifit exceeds 0.3% by weight, it is not only uneconomical, since it cannotbe expected to produce any better results, but it also makes theintergranular corrosion of the galvanizing layer likely to occur easily.If the aluminum content of the bath is lower than 0.1% by weight, nosatisfactory adhesion of the galvanizing layer can be obtained, but ifit exceeds 0.3% by weight, the chromate-coated galvanized surface isundesirably likely to form a black patina when exposed to an environmentof high temperature and humidity.

[0029] The lead-containing galvanized steel sheet exhibits aparticularly beautiful form of regular spangles if it is obtained byemploying a galvanizing bath temperature of 440-500° C. and atemperature of 440-520° C. for the sheet to be immersed into the bath.It is, therefore, advisable to use a lead-containing galvanized steelsheet obtained under the conditions as mentioned if the appearance ofits regular spangles is of particular importance. If the galvanizingbath temperature is lower than 440° C., no beautiful form of regularspangles can be obtained, but if it exceeds 500° C., the alloying ofiron and zinc proceeds to an undesirable extent. If the temperature ofthe sheet to be immersed in the bath is lower than 440° C., no beautifulform of regular spangles can be obtained, but if it exceeds 520° C., itis not only impossible to obtain any beautiful form of regular spangles,but also the galvanizing layer shows an undesirably low adhesion. Thepreferred galvanizing conditions include a galvanizing bath temperatureof 450-480° C. and a sheet immersion temperature of 450-490° C., andmake it possible to obtain a particularly beautiful form of spangles.

[0030] While the process of this invention includes the chromatetreatment of the lead-containing galvanized steel sheet which is carriedout by treating it with the chromate coating solution having a specificcomposition as described above, it is preferable to pre-treat the sheetwith an aqueous alkali solution having a pH of 9 or above, and morepreferably a pH of 9 to 12, in order to remove aluminum from thegalvanized surface.

[0031] If the aqueous alkali solution used for the pre-treatment has apH below 9, the galvanized surface hardly has its aluminum oxidedissolved, but has a lower reactivity with the chromate coatingsolution, and thereby a lower corrosion resistance. If it has a pH ofover 12, an excessively large amount of zinc is etched, and the chromatecoating solution has a lower level of stability. If such pre-treatmentis employed, the galvanized steel sheet is washed with water prior toits chromate treatment.

[0032] Its chromate treatment is carried out for coating it with achromate film formed from a chromate coating solution containinghexavalent and trivalent chromium ions and nitrate ions, and having amolar ratio of trivalent to hexavalent chromium ions and a molar ratioof nitrate to total chromium ions which have both been controlled withinappropriate ranges.

[0033] The trivalent and hexavalent chromium ions in the chromatecoating solution have a molar ratio of from 1/9 to 1/1, and preferablyfrom 1/4 to 2/3 (trivalent/hexavalent). If the molar ratio of thetrivalent/hexavalent chromium ions is smaller than 1/9, the dissolutionof chromium in a corrosive environment is too excessive to sustaincorrosion resistance for a long time, and is also undesirable, as itbrings about environmental pollution. If their molar ratio exceeds 1/1,no satisfactorily improved corrosion resistance can be expected fromhexavalent chromium ions.

[0034] Referring to the mechanism which enables a chromate film toprevent white rust, it is generally understood that hexavalent chromiumions have an inhibitive effect for restraining the corrosion of zinc,and it is also considered that its self-healing effect exhibited by thehexavalent chromium ions eluted from the chromate film and healing anydamaged part thereof contributes to providing an improved anti-whiterust property. If a chromate film is formed from a solution containingonly hexavalent chromium ions, however, the excessive elution ofhexavalent chromium ions in the presence of water causes white rust toform, and it is, therefore, common practice to add trivalent chromiumions in order to prevent the excessive elution of hexavalent chromiumions. For the same purpose, this invention also employs a chromatecoating solution containing trivalent and hexavalent chromium ions inthe molar ratio as mentioned before.

[0035] It is generally necessary for an aqueous solution containing onlytrivalent and hexavalent chromium ions to have a molar ratio oftrivalent/hexavalent chromium ions not exceeding 2/3, so that thetrivalent chromium ions may not settle, but may remain as ions in thesolution. The color tone of its surface is often considered as animportant feature of a lead-containing galvanized steel sheet having abeautiful pattern of spangles, and as the presence of a large amount ofchromium coating produces a yellowish color having an adverse effect onits color tone, there is no alternative but to restrict the amount ofchromium coating even at some sacrifice of corrosion resistance. Thechromate coating solution employed for the purpose of this invention,however, makes it possible to prevent any such undesirable coloringwithout having the amount of chromium coating reduced, since it containsnitric acid as an acid constituent other than chromic acid, and has,therefore, a smaller proportion of hexavalent chromium ions having anadverse effect on the color tone of the galvanized surface, and therebya molar ratio of trivalent/hexavalent chromium ions which is higher than2/3 (but not higher than 1/1, as stated before).

[0036] An aqueous solution of chromic anhydride which has been partiallyreduced by a known reducing agent can be used as a source of chromiumions for the chromate coating solution, while chromium nitrate can beused as a source of trivalent chromium ions.

[0037] The nitrate ions in the chromate coating solution have a molarratio of from 0.1 to 1.6, and preferably from 0.4 to 1.2, to the totalchromium ions. If their molar ratio is smaller than 0.1, no satisfactoryresult can be obtained in restraining any black patina, and if itexceeds 1.6, the chromate coating solution has so high an etching forcethat a sludge is formed by the inclusion of impurity ions, such as Znand Al, and makes it impossible to form a layer having a constantly highcorrosion resistance.

[0038] Nitric acid, chromium nitrate, cobalt nitrate, or zinc nitratecan, for example, be used as a source of nitrate ions for the chromatecoating solution.

[0039] The chromate coating solution may further contain one or morekinds of metal ions selected from among cobalt, nickel, strontium andbarium ions, and having a molar ratio of 0.04 to 0.2 to the totalchromium ions. These metal ions and chromate ions form an insolublecompound which is believed to improve the barrier property of thechromate film and thereby the corrosion resistance of the steel. Iftheir molar ratio is smaller than 0.04, hardly any improved corrosionresistance can be obtained, while if it exceeds 0.2, the chromatecoating solution easily forms a sediment and becomes lower in stability.

[0040] The chromate coating solution is likely to contain Zn, Al, Pb, orother metal ions as unavoidable inclusions by its etching action fromthe galvanizing layer, but they do not adversely affect the advantagesof this invention.

[0041] While several kinds of metal ions have been mentioned, the mostremarkably improved anti-white rust property can be obtained when cobaltions are added. We have experimentally compared a chromate film formedfrom a chromate coating solution containing cobalt ions with a chromatefilm formed from a solution not containing cobalt ions, and found thatthe former contains a larger amount of hexavalent chromium in itsoutermost layer than the latter does, though they are substantiallyequal in their hexavalent chromium content. It is, therefore, obviousthat the chromate film containing cobalt ions exhibits a greaterself-healing effect owing to the hexavalent chromium ions than thechromate film not containing cobalt ions does, and thereby a remarkablyimproved anti-white rust property. Thus, the cobalt ions are the mostpreferable metal ions to be added to the chromate coating solution.

[0042] A basic carbonate, carbonate, or nitrate of a metal can, forexample, be used as a source of its ions for the chromate coatingsolution.

[0043] The chromate coating solution may further contain a silica gel,fumed silica, or other colloidal silica, an aqueous resin, etc., asrequired.

[0044] After the chromate coating solution has been coated on thesurface of the galvanized steel sheet, it is not washed with water, butis dried until a maximum sheet temperature of 40-250° C. is reached. Ifthe sheet temperature is lower than 40° C., the remaining water makes aneasily dissolvable chromate film, and if it exceeds 250° C., thehexavalent chromium ions which are effective for corrosion resistanceare reduced to trivalent chromium ions, and a chromate film formed as ahigh molecule becomes a low molecule, resulting in an undesirablelowering of corrosion resistance.

[0045] The chromate film as formed by coating and drying has a coatingweight of from 5 to 50 mg/m², and preferably from 10 to 30 mg/m², interms of metallic chromium. If its coating weight is less than 5 mg/m²in terms of metallic chromium, no satisfactorily high corrosionresistance can be obtained, while if it exceeds 50 mg/m², the film isdistinctly colored and damages the beautiful surface appearance of thelead-containing galvanized steel sheet.

[0046] Any known method, such as spray or dip coating followed by rollor air squeezing, or roll coating, can be employed for coating thegalvanized surface with the chromate coating solution.

EXAMPLES

[0047] Chromate treatment was given to lead-containing galvanized steelsheets as shown at (A) to (D) below after a part of them had beenpre-treated with an aqueous alkali solution, while no such pre-treatmenthad been given to the rest thereof. The pre-treatment was carried outunder the conditions as stated at (a) or (b) below, and was followed bywashing with a spray of tap water (continued for 10 seconds), and airdrying. The chromate treatment was carried out by roll coating thesheets with chromate coating solutions having the compositions shown inTables 1 to 5 (and basing the control of the chromium coating weight onwet weight), and drying them in a hot-air drying furnace having atemperature of 300° C. and an air flow rate of 2 m/sec. until a maximumsheet temperature of 40-270° C. was reached, whereby chromate-coatedlead-containing galvanized steel sheets were prepared as samples.

[0048] The samples were evaluated for their galvanized surfaceappearance, film appearance, anti-black patina property, and corrosionresistance (anti-white rust property). The evaluation was made of bothof samples as obtained soon after chromate treatment had been started,and samples as obtained after the progress of chromate treatment to someextent (i.e. after the continuous treatment of 20 m² of samples with oneliter of coating solution), and by treatment with the solutioncontaining dissolved zinc, as far as the products of this invention wereconcerned, while two such kinds of samples were evaluated only partly,as far as the comparative cases were concerned. The results are shown inTables 1 to 5 with the composition of the chromate coating solution, thechromium coating weight, etc.

[0049] [Lead-containing galvanized steel sheet]

[0050] (A) A regular-spangled sheet made by dipping a sheet having atemperature of 480° C. in a galvanizing bath containing 0.2% Al and 0.1%Pb, both by weight, and having a temperature of 470° C. (and having acoating weight of 90 g/m²);

[0051] (B) A regular-spangled sheet made by dipping a sheet having atemperature of 440° C. in a galvanizing bath containing 0.2% Al and 0.1%Pb, both by weight, and having a temperature of 440° C. (and having acoating weight of 90 g/m²);

[0052] (C) A regular-spangled sheet made by dipping a sheet having atemperature of 530° C. in a galvanizing bath containing 0.2% Al and 0.1%Pb, both by weight, and having a temperature of 510° C. (and having acoating weight of 90 g/m²); and

[0053] (D) A regular-spangled sheet made by dipping a sheet having atemperature of 430° C. in a galvanizing bath containing 0.2% Al and 0.1%Pb, both by weight, and having a temperature of 430° C. (and having acoating weight of 90 g/m²).

[0054] [Conditions of pre-treatment with an aqueous alkali solution]

[0055] (a) Pre-treatment with an aqueous alkali solution having a pH of9 (spraying at a temperature of 60° C. for 30 seconds);

[0056] and

[0057] (b) Pre-treatment with an aqueous alkali solution having a pH of13 (spraying at a temperature of 50° C. for 3 seconds).

[0058] [Evaluation for properties]

[0059] (1) Appearance of galvanized surface (of spangles)

[0060] The spangles formed on the galvanized surface were visuallyexamined to determine the ratio by area of the frost type spangles, andthe galvanized surface was evaluated for its appearance in accordancewith the following criteria:

[0061] ⊚: The ratio by area is 60% or more;

[0062] ∘: From 40%, inclusive, to 60%, exclusive;

[0063] Δ: From 20%, inclusive, to 40%, exclusive;

[0064] X: Less than 20%.

[0065] (2) Appearance of film

[0066] The value of *b between a sample and an untreated material (bvalue of sample−b value of untreated material) was determined by a colordifference meter, and the film was evaluated for its appearance inaccordance with the following criteria:

[0067] ⊚: Δb is less than 1;

[0068] ∘: Δb is from 1, inclusive, to 3, exclusive;

[0069] Δ: Δb is from 3, inclusive, to 5, exclusive;

[0070] X: Δb is 5 or more.

[0071] (3) Anti-black patina property

[0072] A plurality of testpieces each measuring 70 mm by 150 mm were cutout from each sample, and 5 to 10 pairs of testpieces so prepared thatthe surfaces to be tested of the testpieces in each pair might face eachother, were laid one pair upon another, and packed in a sheet ofvinyl-coated paper. Each package was held between two stainless steelsheets each having a thickness of 10 mm and carrying an acrylic sheetbonded to its inner surface, and after the stainless steel sheets hadbeen fastened together by bolts at their four corners, a load of 0.67kgf.cm² was applied to each package in a humidity test cabinet having atemperature of 50° C. and a relative humidity of 95%, and after 240hours, it was removed from the cabinet and each testpiece was visuallyinspected for any black patina on its surface to be tested. The criteriafor evaluation were as follows:

[0073] ⊚: No black patina was found;

[0074] ∘: A very light gray color was recognized;

[0075] Δ: A black patina was found;

[0076] X: A serious black patina was found.

[0077] (4) Corrosion resistance

[0078] A plurality of testpieces each measuring 70 mm by 150 mm were cutout from each sample, and a salt spray test conforming to JIS Z 2371 wasconducted on each testpiece, and each testpiece was visually examinedfor the area covered by white rust after 96 and 120 hours if it was ofany sample prepared without any pre-treatment, or after 120 and 200hours if it was of any sample prepared after pre-treatment. The criteriafor evaluation were as follows:

[0079] ⊚: No white rust was found;

[0080] ∘: White rust was found in an area of less than 5%;

[0081] Δ: White rust was found in an area of from 5%, inclusive, to 25%,exclusive;

[0082] X: White rust was found in an area of 25% or more.

[0083] Tables 1 to 5 confirm that the chromate-coated lead-containinggalvanized steel sheets manufactured by the process of this inventionhas a galvanized surface with beautiful spangles, and a chromate filmappearance, and anti-black patina and anti-white rust properties whichare all excellent. The samples according to this invention also confirmthat the restrained etching effect on galvanized steel sheets makes theprocess highly suitable for continuous operation. On the other hand, thecomparative samples are inferior in any of galvanized surface, orchromate film appearance, and anti-black patina and anti-white rustproperties. TABLE 1 [3] [1] [2] Cr³⁺/Cr⁶⁺ NO³⁻/T-Cr [4] Zn²⁺ [6] [7][11] [12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l) (° C.) *4 [8] [9] [10] *5*6 1a A Not given 1/4 0.8 — — 0 60 15 ⊚ ⊚ ⊚ ⊚/◯ I 1b A Not given 1/4 0.8— — 0.4 60 17 ⊚ ⊚ ⊚ ⊚/◯ I 2a B Not given 1/4 0.8 — — 0 60 16 ◯ ⊚ ⊚ ⊚/◯ I2b B Not given 1/4 0.8 — — 0.4 60 15 ◯ ⊚ ⊚ ⊚/◯ I 3a C Not given 1/4 0.8— — 0 60 16 X ⊚ ⊚ ⊚/◯ I 3b C Not given 1/4 0.8 — — 0.4 60 14 X ⊚ ⊚ ⊚/◯ I4a D Not given 1/4 0.8 — — 0 60 16 Δ ⊚ ⊚ ⊚/◯ I 4b D Not given 1/4 0.8 —— 0.4 60 18 Δ ⊚ ⊚ ⊚/◯ I 5a A Not given 1/4 0.8 Co:0.04 — 0 60 17 ⊚ ⊚ ⊚⊚/⊚ I 5b A Not given 1/4 0.8 Co:0.04 — 0.4 60 15 ⊚ ⊚ ⊚ ⊚/⊚ I 6a A Notgiven 1/1 1.6 Co:0.20 — 0 150 47 ⊚ ◯ ⊚ ⊚/⊚ I 6b A Not given 1/1 1.6Co:0.20 — 0.8 150 49 ⊚ ◯ ⊚ ⊚/⊚ I 7a A Not given 1/2 1.1 Co:0.10 — 0 8020 ⊚ ⊚ ⊚ ⊚/⊚ I 7b A Not given 1/2 1.1 Co:0.10 — 0.6 80 18 ⊚ ⊚ ⊚ ⊚/⊚ I 8aA Not given 1/9 0.1 Ni:0.06 — 0 100 21 ⊚ ⊚ ⊚ ⊚/◯ I 8b A Not given 1/90.1 Ni:0.06 — 0.8 100 22 ⊚ ⊚ ⊚ ⊚/◯ I 9a A Not given 1/5 1.0 Co:0.10 — 080 8 ⊚ ⊚ ⊚ ⊚/◯ I 9b A Not given 1/5 1.0 Co:0.10 — 0.6 80 9 ⊚ ⊚ ⊚ ⊚/◯ I

[0084] TABLE 2 [3] [1] [2] Cr³⁺/Cr⁶⁺ NO³⁻/T-Cr [4] Zn²⁺ [6] [7] [11][12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l) (° C.) *4 [8] [9] [10] *5 *6 10aA Not given 1/5 1.0 St:0.10 — 0 80 25 ⊚ ⊚ ⊚ ⊚/◯ I 10b A Not given 1/51.0 St:0.10 — 0.4 80 22 ⊚ ⊚ ⊚ ⊚/◯ I 11a A Not given 1/5 1.0 Ba:0.10 — 080 15 ⊚ ⊚ ⊚ ⊚/◯ I 11b A Not given 1/5 1.0 Ba:0.10 0.3 80 17 ⊚ ⊚ ⊚ ⊚/◯ I12a A Not given 1/8 0.2 Co:0.05 SiO₂:4 *7 0 60 20 ⊚ ⊚ ⊚ ⊚/◯ I 12b A Notgiven 1/8 0.2 Co:0.05 SiO₂:4 *7 0.2 60 22 ⊚ ⊚ ⊚ ⊚/◯ I 13a A Not given1/4 0.8 Co:0.08 resin:5 *8 0 230 32 ⊚ ⊚ ⊚ ⊚/⊚ I 13b A Not given 1/4 0.8Co:0.08 resin:5 *8 0.4 230 31 ⊚ ⊚ ⊚ ⊚/⊚ I 14  A Not given 1/19 0.8Co:0.05 — 0 40 19 ⊚ ⊚ ◯ X/X C 15  A Not given 1/4 — — — 0 60 25 ⊚ ◯ XX/X C 16a A Not given 1/1 2.0 Co:0.05 — 0 150 30 ⊚ ⊚ ⊚ X/X C 16b A Notgiven 1/1 2.0 Co:0.05 — 3.5 150 28 ⊚ ⊚ ⊚ X/X C 17  A Not given 1/3 0.4Co:0.08 — 0 270 20 ⊚ ◯ ⊚ X/X C 18  A Not given 1/2 — Co:0.10 PO₄ ³⁻:0.3*9 0 80 18 ⊚ ⊚ X ⊚/⊚ C 19  A Not given 1/2 — Co:0.10 SO₄ ²⁻:0.3 *9 0 8048 ⊚ X X ⊚/⊚ C 20  A Not given 1/2 — Co:0.05 F⁻:0.2 *9 0 80 30 ⊚ ◯ X ⊚/⊚C 21 A Not given 1/1 2.0 Co:0.05 — 0 80 72 ⊚ Δ ⊚ Δ/⊚ C

[0085] TABLE 3 [3] [1] [2] Cr³⁺/Cr⁶⁺ NO³⁻/T-Cr [4] Zn²⁺ [6] [7] [11][12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l) (° C.) *4 [8] [9] [10] *5 *6 22aA a 1/4 0.8 — — 0 40 18 ⊚ ⊚ ⊚ ◯/◯ I 22b A a 1/4 0.8 — — 0.3 40 17 ⊚ ⊚ ⊚◯/◯ I 23a B a 1/4 0.8 — — 0 40 19 ◯ ⊚ ⊚ ◯/◯ I 23b B a 1/4 0.8 — — 0.3 4017 ◯ ⊚ ⊚ ◯/◯ I 24a C a 1/4 0.8 — — 0 40 20 X ⊚ ⊚ ◯/◯ I 24b C a 1/4 0.8 —— 0.3 40 18 X ⊚ ⊚ ◯/◯ I 25a D a 1/4 0.8 — — 0 40 19 Δ ⊚ ⊚ ◯/◯ I 25b D a1/4 0.8 — — 0.3 40 20 Δ ⊚ ⊚ ◯/◯ I 26a A a 1/4 0.8 Co:0.04 — 0 60 21 ⊚ ⊚⊚ ⊚/⊚ I 26b A a 1/4 0.8 Co:0.04 — 0.4 60 19 ⊚ ⊚ ⊚ ⊚/⊚ I 27a A a 1/1 1.6Co:0.20 — 0 150 32 ⊚ ⊚ ⊚ ⊚/⊚ I 27b A a 1/1 1.6 Co:0.20 — 0.8 150 35 ⊚ ⊚⊚ ⊚/⊚ I

[0086] TABLE 4 [3] [1] [2] Cr³⁺/Cr⁶⁺ NO³⁻/T-Cr [4] Zn²⁺ [6] [7] [11][12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l) (° C.) *4 [8] [9] [10] *5 *6 28aA a 1/2 1.1 Co:0.10 — 0 80 18 ⊚ ⊚ ⊚ ⊚/⊚ I 28b A a 1/2 1.1 Co:0.10 — 0.680 19 ⊚ ⊚ ⊚ ⊚/⊚ I 29a A a 1/9 0.1 Ni:0.06 — 0 100 20 ⊚ ⊚ ⊚ ⊚/◯ I 29b A a1/9 0.1 Ni:0.06 — 0.8 100 22 ⊚ ⊚ ⊚ ⊚/◯ I 30a A a 1/5 1.0 Co:0.10 — 0 808 ⊚ ⊚ ⊚ ⊚/◯ I 30b A a 1/5 1.0 Co:0.10 — 0.6 80 9 ⊚ ⊚ ⊚ ⊚/◯ I 31a A a 1/51.0 Sr:0.10 — 0 80 22 ⊚ ⊚ ⊚ ⊚/◯ I 31b A a 1/5 1.0 Sr:0.10 — 0.6 80 21 ⊚⊚ ⊚ ⊚/◯ I 32a A a 1/5 1.0 Ba:0.10 — 0 80 18 ⊚ ⊚ ⊚ ⊚/◯ I 32b A a 1/5 1.0Ba:0.10 — 0.6 80 19 ⊚ ⊚ ⊚ ⊚/◯ I 33a A a 1/8 0.2 Co:0.05 SiO₂:4 *7 0 6010 ⊚ ⊚ ⊚ ⊚/◯ I 33b A a 1/8 0.2 Co:0.05 SiO₂:4 *7 0.2 60 11 ⊚ ⊚ ⊚ ⊚/◯ I

[0087] TABLE 2 [3] [1] [2] Cr³⁺/Cr⁶⁺ NO³⁻/T-Cr [4] Zn²⁺ [6] [7] [11][12] No. *1 *2 *3 *3 /T-Cr *3 [5] (g/l) (° C.) *4 [8] [9] [10] *5 *6 34aA a 1/4 0.8 Co:0.08 resin:5 *8 0 230 41 ⊚ ⊚ ⊚ ⊚/⊚ I 34b A a 1/4 0.8Co:0.08 resin:5 *8 0.4 230 42 ⊚ ⊚ ⊚ ⊚/⊚ I 35  A a  1/19 0.8 Co:0.05 — 040 21 ⊚ ⊚ ◯ X/X C 36  A a 1/4 — — — 0 60 24 ⊚ ◯ X X/X C 37a A b 1/1 2.0Co:0.05 — 0 150 29 ⊚ ⊚ ⊚ X/X C 37b A b 1/1 2.0 Co:0.05 — 3.5 150 27 ⊚ ⊚⊚ X/X C 38  A a 1/3 0.4 Co:0.08 — 0 270 20 ⊚ ◯ ⊚ X/X C 39  A a 1/2 —Co:0.10 PO₄ ³⁻:0.3 *9 0 80 18 ⊚ ⊚ X ⊚/◯ C 40  A a 1/2 — Co:0.10 SO₄²⁻:0.3 *9 0 80 48 ⊚ X X ⊚/◯ C 41  A a 1/2 — Co:0.05 F⁻:0.2 *9 0 80 30 ⊚◯ X ⊚/◯ C 42  A a 1/1 2.0 Co:0.05 — 0 80 72 ⊚ Δ ⊚ ⊚/Δ C

INDUSTRIAL UTILITY

[0088] This invention enables the manufacture of chromate-coatedlead-containing galvanized steel sheets which are suitable as buildingmaterials, or materials for electric appliances.

1. A process for manufacturing a chromate-coated lead-containinggalvanized steel sheet having excellent anti-black patina and anti-whiterust properties, characterized in that a lead-containing galvanizedsteel sheet obtained by plating a steel sheet in a galvanizing bathcontaining 0.05 to 0.3% by weight of Pb and 0.1 to 0.3% by weight of Alis coated with a chromate coating solution containing hexavalent andtrivalent chromium ions and nitrate ions in such proportions that thetrivalent chromium ions have a molar ratio of 1/9 to 1/1 to thehexavalent chromium ions, while the nitrate ions have a molar ratio of0.1 to 1.6 to the total of the chromium ions, and that the sheet isdried at a sheet temperature of 40-250° C. without being washed withwater, whereby a chromate film having a coating weight of 5 to 50 mg/m²in terms of metallic chromium is formed thereon.
 2. A process formanufacturing a chromate-coated lead-containing galvanized steel sheethaving excellent anti-black patina and anti-white rust properties as setforth in claim 1 , wherein the lead-containing galvanized steel sheet isobtained by employing a temperature of 440-500° C. for the galvanizingbath and a temperature of 440-520° C. for the sheet to be immersed intothe bath.
 3. A process for manufacturing a chromate-coatedlead-containing galvanized steel sheet having excellent anti-blackpatina and anti-white rust properties as set forth in claim 1 or 2 ,wherein the chromate coating solution contains one or more kinds ofmetal ions selected from among cobalt, nickel, strontium and bariumions, and having a total molar ratio of 0.04 to 0.2 to the total of thechromium ions.
 4. A process for manufacturing a chromate-coatedlead-containing galvanized steel sheet having excellent anti-blackpatina and anti-white rust properties as set forth in claim 1 or 2 ,wherein the chromate coating solution contains cobalt ions having amolar ratio of 0.04 to 0.2 to the total of the chromium ions.
 5. Aprocess for manufacturing a chromate-coated lead-containing galvanizedsteel sheet having excellent anti-black patina and anti-white rustproperties, characterized in that a lead-containing galvanized steelsheet obtained by plating a steel sheet in a galvanizing bath containing0.05 to 0.3% by weight of Pb and 0.1 to 0.3% by weight of Al is treatedwith an aqueous alkali solution having a pH of 9 or above, and thatafter it is washed with water, it is coated with a chromate coatingsolution containing hexavalent and trivalent chromium ions and nitrateions in such proportions that the trivalent chromium ions have a molarratio of 1/9 to 1/1 to the hexavalent chromium ions, while the nitrateions have a molar ratio of 0.1 to 1.6 to the total of the chromium ions,and that the sheet is dried at a sheet temperature of 40-250° C. withoutbeing washed with water, whereby a chromate film having a coating weightof 5 to 50 mg/m² in terms of metallic chromium is formed thereon.
 6. Aprocess for manufacturing a chromate-coated lead-containing galvanizedsteel sheet having excellent anti-black patina and anti-white rustproperties as set forth in claim 5 , wherein the lead-containinggalvanized steel sheet is obtained by employing a temperature of440-500° C. for the galvanizing bath and a temperature of 440-520° C.for the sheet to be immersed into the bath.
 7. A process formanufacturing a chromate-coated lead-containing galvanized steel sheethaving excellent anti-black patina and anti-white rust properties as setforth in claim 5 or 6 , wherein the chromate coating solution containsone or more kinds of metal ions selected from among cobalt, nickel,strontium and barium ions, and having a total molar ratio of 0.04 to 0.2to the total of the chromium ions.
 8. A process for manufacturing achromate-coated lead-containing galvanized steel sheet having excellentanti-black patina and anti-white rust properties as set forth in claim 5or 6 , wherein the chromate coating solution contains cobalt ions havinga molar ratio of 0.04 to 0.2 to the total of the chromium ions.